1. Field of the Invention
The present invention relates to a camera system for calculating a target driving amount by using the focus information of an image-taking optical system while a focusing lens is driven.
2. Description of Related Art
A TTL (through the lens) type camera for performing focusing of the image-taking optical method by using a phase difference detection system is conventionally known, wherein the focus detection operation is repeatedly performed while the focusing lens is driven so as to shorten the time to be required for focusing (re-focus detection operation during driving, hereinafter, referred to as “overlap operation”).
An automatic focus controller is disclosed in Japanese Patent Publication No. H04 (1992)-10051, which performs the focus detection repeatedly while stopping and moving an image taking lens, and moves the image taking lens to an in-focus position while correcting the driving amount of the image taking lens.
Herein, it will be considered that the overlap operation described above is applied to the following camera system. This camera system captures an optical image formed by the image-taking optical system by using an image sensor provided in a camera body. In addition, the camera system obtains a defocus amount based on the signal showing the amount of detected light from the image sensor, and transmits the defocus amount to the side of an interchangeable lens. The interchangeable lens calculates the driving amount of the focusing lens based on the received defocus amount, and moves the focusing lens to a target position (in-focus position).
For example, Japanese Patent Laid-Open No. H05(1993)-196863 discloses a technique wherein the driving of the focusing lens is controlled by transmitting the defocus amount from the side of the camera body to the side of the interchangeable lens, and converting the defocus amount into the lens driving amount on the side of the interchangeable lens.
Herein, the calculated defocus amount can be converted into the driving amount of the focusing lens at the side of the camera body, and the driving amount can be transmitted to the side of the interchangeable lens. The focusing lens can be driven based on the lens driving amount received at the side of the interchangeable lens. However, in this case, it is necessary to acquire various kinds of optical data (sensitivity coefficient or the like corresponding to a focal length and lens position) of the interchangeable lens by communications, which is required when the camera body converts the defocus amount into the lens driving amount. Therefore, the camera system for transmitting the defocus amount to the side of the interchangeable lens has the advantages for which the number of communications (transmitting and receiving data) can be remarkably reduced compared with the case that the lens driving amount is transmitted to the side of the interchangeable lens, and the communications load can be reduced.
Herein, in the camera system for transmitting the defocus amount to the interchangeable lens, the focusing lens is moved from the time when detecting the amount of light necessary to calculate the defocus amount to the time when calculating the defocus amount and transmitting the defocus amount to the interchangeable lens while the overlap operation is performed, that is, in the time obtained by adding the charge storage time with respect to the object light, the calculation time and the communications time, and even during the time before converting the defocus amount received at the side of the interchangeable lens into the lens driving amount.
The relationship thereof will be described with reference to FIG. 11. FIG. 11 shows one example of the relationship between the lens driving speed and the lens position (pulse count value) from the start of the driving of the focusing lens until the attainment of the target position. Though timings shown by A to E in FIG. 11 show the timings of the operations to be described below schematically, these timings do not necessarily coincide with those in FIG. 11.
In the driving of the focusing lens, the focusing lens is firstly accelerated immediately after the start of the driving, and the lens driving speed is increased up to a constant speed. The deceleration is started when the lens position is set in a predetermined position after the lens driving speed is set to a constant speed, and the focusing lens is stopped at a target position. Herein, the overlap operation is performed only at the time at which the lens driving speed is set to a constant speed. The reason is that the overlap operation causes errors in the focus detection while the focusing lens is accelerated or is decelerated.
The charge storage operation is started in the image sensor of the side of the camera body for calculating the defocus amount when the overlap operation is performed (timing A in FIG. 11), and the charge storage operation is ended after passage of a predetermined time (timing B in FIG. 11). Herein, a storage center timing means the intermediate between the timing at which the charge storage is started and the timing at which the charge storage is ended.
The defocus amount is then calculated based on the light amount signal (accumulated charge) readout from the image sensor (timing C in FIG. 11), and data concerning the defocus amount is transmitted to the side of the interchangeable lens. The interchangeable lens receives the data concerning the defocus amount transmitted from the camera body (timing D in FIG. 11), converts the defocus amount into the lens driving amount, and starts the driving of the focusing lens (timing E in FIG. 11).
As shown in FIG. 11, the focusing lens is moved between the storage center timing and the timing E at which the lens drive is started. Therefore, various kinds of optical data (sensitivity coefficient or the like corresponding to the focal length and lens position) of the lens used for converting the defocus amount into the lens driving amount may be changed between the storage center timing and the timing E. The lens driving amount may not be accurately calculated when various kinds of optical data are acquired after the defocus amount is received.
When the focusing lens is driven by the lens driving amount calculated based on the optical data which does not correspond to the present position of the focusing lens, the focusing lens exceeds the in-focus position by only the extent that the focusing lens is moved. In this case, it is necessary to correct the lens driving amount appropriately for stopping the focusing lens at the in-focus position.